It is well known that an aging lens precludes or diminishes a person's capacity to focus on near objects. Focus is accomplished by changing the curvature of the lens and the angular relation of the eyes to each other in accordance with the distance of the observed object to the eye.
The change in curvature of the lens, called "Accommodation" is controlled by the circular fibers of the ciliary muscle which is an annulus that is connected to the lens by the zonules. When the ciliary muscle is relaxed, the lens is flattened, i.e., it has less curvature. Thus, distant objects are in focus.
On the other hand, contraction of the ciliary muscle will cause the zonules to relax and the lens to thicken, i.e., it has more curvature, thereby shortening its focal distance to accommodate the viewing of a near object.
Accommodation causes the angular relation of the eyes to change such as by rotating them inwardly so that they turn toward the near object and so that they turn down. The inward rotation which is caused by the medial rectus muscles is called "Convergence." The downward turning which is caused by the inferior rectus muscles does not have a specific name.
The amount of Convergence caused by an Accommodating lens is called the Accommodation--Convergence Ratio: ##EQU1##
where Accommodation is measured in diopters and Convergence is measured in meter angles (m.a.).
The Accommodation--Convergence Ratio which is about 4:1 is constant throughout life.
On the other hand, when the medial rectus muscle causes the eye to turn toward a near object, the lens accommodates a given amount. This is the Convergence--Accommodation Ratio: EQU C/A
The Convergence--Accommodation Ratio varies over a lifetime. For young people, the ratio is about 4 m.a. per diopter. However, as a result of ageing the lens gradually becomes less elastic and the ciliary muscle weakens so that the ciliary muscle is not strong enough to cause the lens to adequately accommodate so that when the eyes converge 4 m.a., the accommodation is less than one diopter and there is a failure to focus. The reduction of the ability of the lens to accommodate is called presbyopia.
Therefore, as a person ages, more stimulation must be applied to the ciliary muscle so that it can contract and maintain the 1:4 ratio.
As explained in Kamerling U.S. Pat. No. 4,603,697, which issued Aug. 5, 1986, the content of which is hereby incorporated in its entirety by reference, the inability of the eye to focus on near objects can be compensated for by the use of an implanted electrode that can assist the eye to maintain focus. The electrode stimulates the ciliary muscle to constrict and thereby thicken the lens to increase its curvature so that it focuses on a near object thereby keeping the presbyopes in focus.
The system described in the Kamerling patent relies upon an external or internal source of power which must be energized to stimulate the ciliary muscle. That patent discloses that the stimulation can be achieved by using radio signals which are detected by an antenna.
It would be desirable if the lens could be automatically focused without external stimulation when a person changes the focus to read or see a near object.